1. Field of the Invention
The subject invention relates to an assembly for sensing moisture on a sheet of glass and, more particularly, to a rain sensor for detecting rain on the windshield of an automotive vehicle to turn on the wipers.
2. Description of the Prior Art
The sensing of rain or moisture on the windshield of a vehicle for automatically controlling wiping systems has become a popular driver convenience item in the last several years. With such automation, drivers can be more focused on the road than on the manual setting of the windshield wiper controls. Traditional wiper controls are typically preset to one of several speeds, and/or the wipe rate interval is set according to the rainfall rate. Of course as the rainfall (rate) increases or decreases the proper setting to maintain a nominally clear windshield varies, and, therefore, necessitates frequent driver adjustment.
Such fully automatic, rain sensing, wiper control systems continually monitor the rain that is falling on the windshield, and respond by adjusting the wiping interval to match the rain-fall rate.
Several methods have been used to achieve such automatic wiper control and, for the most part, such methods differ only in the rain sensing detectors that are employed. That is, the rain sensing element is generally adapted to work with or control existing interval (timed) wiping systems. The most common rain or moisture sensing detectors have been based on either the optical or the electrical properties of water.
The electrical rain sensors were used in many of the early automatic wiping systems and in general sense the presence of rain droplets on the outer surface of the windshield by resistive or capacitive means.
The resistive sensors usually employ a conductive detecting grid or interleaved electrodes plated onto the glass surface. The small gaps in such sensing arrays are bridged by water drops to trigger the wiping mechanism. The partly conducting nature of rain water renders such sensors very sensitive, but the problems related to the attachment of the sensor electrodes to the windshield have not been successfully resolved. Furthermore, the slight difference in surface smoothness and surface tension between the electrode material and the surrounding glass has been shown to interfere with wiping efficiency. The life of the electrode has also been a problem with such exterior sensors, as the rubber wiper blades can be quite abrasive if they become loaded with road grime.
The capacitive sensors such as those typified by U.S. Pat. No. 4,703,237 permit the sensing electrodes to be positioned on the inside surface of the windshield while retaining small droplet sensitivity. A limitation of such sensors is that they utilize the high dielectric constant (80) of water to detune a bridge or resonant circuit which requires the sensing electrode (array) to be relatively large to provide the requisite sensitivity. The disposition of suitably large structures on the windshield may interfere with driver vision and are not aesthetically acceptable to the industry.
Optical sensors of various types have gained favor in recent years, but in general suffer from the same limitations as the electrical sensing systems. Conventional optical rain sensors commonly work on the extinction principle, and are designed to be attached to the inside surface of the windshield. Typical of such devices is the rain sensor described in U.S. Pat. No. 4,960,996. The primary reason that such optical sensors have not been totally successful is that they are restricted to sensing a small area of the windshield, generally within the wiped field. Restricting the field of view to a few square inches of glass area has been shown to present problems in certain rain situations. For example, fine and evenly dispersed droplets are easily sensed, but large rain drops with a mean spacing of several inches are not always detected by such small area sensors. That is, if one or more droplets do not fall within the small sensing area of such optical detectors, the device will obviously not trigger the wipers. Increasing the viewing area of traditional optical rain sensors is not practical, because as noted, these devices are invariably designed for "on glass" mounting. As in the case of electrical sensors, enlarging the sensor to monitor a physically larger area of the glass is impractical because of the intrusive nature of such hardware.
Another class of optical rain sensor which does allow for off the glass mounting, and a greater field of view, is based on the back-scatter principle. These sensors aim one or more light sources at the outer windshield surface through the glass and detect the amount of radiation back-scattered toward one or more electro-optic detectors. Rapid changes in ambient lighting conditions often confuse such sensors as do moving patterns of light or shadows that fall on the windshield. Rain sensors based on diffuse back scatter light are typified by U.S. Pat. Nos. 5,673,744, 5,416,318 and 5,264,691.
Virtually all existing rain sensors are prone to false trigger and initiate "phantom wipes" that can be annoying or distracting to the driver. Such false triggering may be due to a variety of reasons but is most often due to dirt on the windshield which may mimic rain for the sensor, or by moving light patterns as noted above. Also, existing optical rain sensors are often overly sensitive to excessive ambient light intensity which may saturate the detectors and under some circumstances, desensitize the detector to rain drops.
The U.S. Pat. No. 4,867,561 to Fujii, et al, discloses an imaging rain sensor, i.e., that, a multi element extinction rain sensor. As noted in this patent, the total reflection from the outer surface of the windshield is reduced by the presence of water drops, as they couple light energy out of the glass. The use of a collimating lens allows the glass reflection to be location specific. The use of first and second light sources, coupled with first and second detectors, is necessary to compensate for ambient light variations. That is, certain photo detector elements are relegated to certain or different functions. In order to achieve the requisite alignment for the extinction principle to work properly, the illumination sources have to be carefully positioned relative to the windshield and the detector, as described in Col. 6, lines 44-65 of the Fujii '561 patent. This critical alignment requirement necessitates mounting the detector assembly close to or in actual contact with the windshield and such close mounting limits the effective field or area of view. The successful implementation of a large area rain sensor based on the extinction principle would require a large linear array of I.R. emitting diodes to effectively illuminate the sensing field.
Experiments in human factors with artificial rain making equipment have been used to determine the optimal wiping intervals and rain [trigger] sensitivity of fully automatic rain sensors. One aspect of these experiments established the minimum useful area for a rain sensor, or the smallest area on the windshield that could be monitored by the active rain sensing element. The importance of this "area parameter" is generally not widely recognized, but is intuitively obvious upon careful analysis. Small rain droplets [and mist] are generally evenly and densely distributed over the windshield, while larger drops may be much more widely spaced and at least locally, not evenly distributed. That is, a relatively small observation or sensing area may not allow a rain detector to trigger on larger water droplets, e.g., a mean spacing greater than 50 mm. Existing `on the glass` rain sensors are generally limited to measuring areas of under 500 mm squared because larger sensing areas, which require bulkier housings, become physically intrusive and thereby tend to obscure critical portions of the windshield. The human factors experiments suggest that an optimal active sensing area of greater than 6500 mm squared would be required to accommodate all rain conditions and driver expectations. Naturally, attaching a sensor of 50 mm.times.130 mm [nom. 2 in..times.5 in.] to the windshield is impractical, and particularly so when the sensor needs to be located within the wiped portion of the glass.
This paradox of needing to examine a substantial area on the glass for incidence of rain, while at the same time disallowing the attachment of physically large sensors to the glass presents a problem to be solved by a rain sensor.